Generally, a faceplate panel for CRTs has a convex-shaped lens with curved inner and outer surfaces. The convex lens-shaped panel is advantageous in various aspects such as convenience in formation, stability in strength and adaptability for shadow mask application.
However, to the eye of the viewer, it is desirable that the screen image be displayed substantially flat. For this purpose, several attempts have been made to form the inner and outer surfaces of the faceplate panel with a flat shape while maintaining normal display characteristics of the CRT. It is found that when a flat panel is employed for the display screen problems occur in both the convergence characteristics of electron beams and in the strength of a shadow mask. For instance, because the flat-shaped inner surface of the panel is naturally formed with a flat phosphor screen, it becomes difficult to deflect three electron beams of red R, green G and blue B on correct phosphors on the phosphor screen. Furthermore, because the shadow mask facing the flat-shaped inner surface of the panel should also be flat, a desirable shadow mask strength cannot be achieved using the common shadow mask forming technique.
In addition, there is a problem with the flat-panel CRT from the standpoint of the viewer. When the viewer watches a monitor with a flat-shaped panel, the viewer feels that the screen image is sunken at its center portion while protruded at its peripheral portion.
Therefore, it is preferably that the outer surface of the panel is flat whereas the inner surface of the panel is curved.
In such a faceplate panel, as the overall curvature radius of the inner curved surface of the panel becomes smaller, the panel is more easily produced and the shadow mask has a more stable structure capable of reducing a doming phenomenon. However, when the curvature radius falls short of a minimum effective value, the peripheral portion of the panel bears an undesirably large thickness and this results in poor production efficiency as well as high production cost. Furthermore, the transmission rate of the peripheral portion becomes poor due to its large thickness, causing brightness failure.
In order to overcome such problems, various techniques are proposed for a one-sided flat panel CRT application. For example, some techniques are disclosed in Japanese Patent Laid Open Publication No. 6-36710 and No. 6-644926. However, the technical details are not specified for preserving the structural strength of the shadow mask which should be re-designed pursuant to the curvature radii varying at different positions of the inner curved surface of the panel.
Furthermore, the prior art does not discriminate the desired thickness ratios of a diagonal portion of the panel to the peripheral portion for minimizing distortion of the screen image. Therefore, when the CRT panel is manufactured on the basis of the above-identified techniques, the aforementioned problems remain unsolved.
In the usual sized flat-panel 21-inch, 25-inch and 29-inch CRTs, the thickness ratios of the peripheral portion of the panel to the center portion are 3.13, 2.91 and 2.72, respectively. These ratios are so high that they result in poor production efficiency as well as brightness failure.